Electrical impedance mammography (EIM), or Electrical impedance imaging (EII), also referred to as electrical impedance tomography (EIT), electrical impedance scanner (EIS) and applied potential tomography (APT), is an imaging technique that is particularly used in medical applications.
The technique images the spatial distribution of electrical impedance inside an object, such as the human body. The technique is attractive as a medical diagnostic tool because it is non-invasive and does not use ionizing radiation as in X-ray tomography or the generation of strong, highly uniform magnetic fields as in Magnetic Resonance Imaging (MRI).
Typically a two-dimensional (2D) or three-dimensional (3D) array of evenly spaced electrodes is attached to the object to be imaged about the region of interest. Either input voltages are applied across pairs of ‘input’ electrodes and output electric currents are measures at the ‘output’ electrodes or input electric currents are applied between pairs of ‘input’ electrodes and output voltages are measured between at the ‘output’ electrodes or between pairs of output electrodes. For example, when a very small alternating electric current is applied between a pair of ‘input’ electrodes, the potential difference between all other pairs of ‘output’ electrodes is measured. The current is then applied between a different pair of ‘input’ electrodes and the potential difference between all other pairs of ‘output’ electrodes is measured. An image is constructed using an appropriate image reconstruction technique.
Spatial variations revealed in electrical impedance images may result from variations in impedance between healthy and non-healthy tissues, variations in impedance between different tissues and organs or variations in apparent impedance due to anisotropic effects resulting for example from muscle alignment.
Tissue or cellular changes associated with cancer cause significant localized variations in electrical impedance and can be imaged. WO 00/12005 discloses an example of electrical impedance imaging apparatus that can be used to detect breast carcinomas or other carcinomas.
A most common problem associated with impedance imaging is that there are unexpected and unknown electrical impedances present other than those provided by the object to be imaged. The unexpected and unknown electrical impedances may arise from the imaging electrode sensor system and the associated changes in electrical impedance may be much greater than the internal electrical impedance of the object and its changes and may therefore mask local variations in the internal electrical impedance of the object or changes in such impedances. For example, the imperfect contact between an electrode and the object to be imaged introduces an impedance at each electrode. Such an impedance may vary in an unknown manner from electrode to electrode or vary over time or varying depending on how the electrode has been attached to the object. This can introduce significant unpredictable errors into the produced image and can therefore result in an unrepeatable image being produced. This will prevent the accurate detection and monitoring of the development of, for example, non-healthy tissue.